Steel sheet, coated steel sheet, method for producing hot-rolled steel sheet, method for producing cold-rolled full hard steel sheet, method for producing steel sheet, and method for producing coated steel sheet

ABSTRACT

A steel sheet is provided, having a tensile strength of 340 MPa or more, a particular composition and a steel structure in which ferrite has an average crystal grain size of 5 m or more and 25 m or less, 40% or more of cementite in terms of area fraction precipitates in ferrite grain boundaries, and the ferrite has a texture in which an inverse intensity ratio of γ-fiber to α-fiber is 0.8 or more and 7.0 or less.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2017/008956, filedMar. 7, 2017, which claims priority to Japanese Patent Application No.2016-070748, filed Mar. 31, 2016 and Japanese Patent Application No.2016-232542, filed Nov. 30, 2016, the disclosures of each of theseapplications being incorporated herein by reference in their entiretiesfor all purposes.

FIELD OF THE INVENTION

The present invention relates to a steel sheet, a coated steel sheet, amethod for producing a hot-rolled steel sheet, a method for producing acold-rolled full hard steel sheet, a method for producing a steel sheet,and a method for producing a coated steel sheet.

BACKGROUND OF THE INVENTION

Cold-rolled steel sheets are used as the raw material for variousstructures due to their good formability. Typically, cold-rolled steelsheets are press-formed into three-dimensional structures. Furthermore,three-dimensional structures are often joined with one another to form acomplicated three-dimensional structure. Thus, cold-rolled steel sheetsare required to have excellent workability.

Patent Literature 1 proposes one example of a cold-rolled steel sheethaving excellent workability, which is a high-workability cold-rolledsteel sheet having low C, Mn, Al, and N contents and being prepared by aproduction method that involves cold-rolling a steel sheet at a rollingreduction ratio of 50% or more and then regulating the coolingconditions and the overaging conditions after annealing while alsoregulating the skinpass rolling reduction ratio.

Patent Literature 2 discloses a method for producing a cold-rolled steelsheet having excellent workability, the method including continuouslycasting a steel with prescribed C, Mn, S, O, and B contents underprescribed conditions, and then performing hot rolling, cold rolling,and continuous annealing.

Patent Literature 3 discloses a technique of producing a cold-rolledsteel sheet having excellent workability, the method includinghot-rolling and cold-rolling a steel having prescribed C, Si, Mn, P, Al,and N contents, and then quickly heating and quickly cooling theresulting rolled sheet during continuous annealing.

PATENT LITERATURE

PTL 1: Japanese Unexamined Patent Application Publication No. 61-124533

PTL 2: Japanese Unexamined Patent Application Publication No. 2-267227

PTL 3: Japanese Unexamined Patent Application Publication No. 7-216459

SUMMARY OF THE INVENTION

However, according to the production method described in PatentLiterature 1, the increase in skinpass rolling reduction ratio cannot beavoided, and degradation of workability due to skinpass rolling cannotbe avoided.

According to the production method described in Patent Literature 2,although the size of MnS is controlled through oxide inclusions, 60 ppmor more of oxygen must be contained. Thus, large quantities of oxideinclusions occur, and, in press-forming, cracking occurs from theinclusions as the starting points.

According to the production method described in Patent Literature 3, theheat transfer in the steel cannot be made uniform throughout the entiresteel sheet, and the workability is improved only in some part of thesteel sheet.

Moreover, although all of the patent literatures disclose that ductilityis particularly excellent among various properties related toworkability, the planar anisotropy of YP is not considered. Moreover,the surface properties of the steel sheet are not considered.

The present invention has been developed under the above-describedcircumstances, and an object thereof is to provide a steel sheet and acoated steel sheet that have a tensile strength (TS) of 340 MPa or more,excellent workability, excellent YP planar anisotropy, and excellentsurface properties, and methods for producing the steel sheet and thecoated steel sheet. Another object is to provide a method for producinga hot-rolled steel sheet and a method for producing a cold-rolled fullhard steel sheet necessary for obtaining the steel sheet and coatedsteel sheet.

For the purposes of the present invention, excellent workability meansthat the product, TS×El (El denotes the total elongation), is 13000MPa·% or more. Moreover, excellent YP planar anisotropy means that thevalue of the index of the planar anisotropy of YP, |ΔYP|, is 30 MPa orless. Here, |ΔYP| is determined by formula (1) below:

|ΔYP|=(YPL−2×YPD+YPC)/2  (1)

where YPL, YPD, and YPC respectively represent values of YP measuredfrom JIS No. 5 test pieces taken in three directions, namely, therolling direction (L direction) of the steel sheet, a direction (Ddirection) 45° with respect to the rolling direction of the steel sheet,and a direction (C direction) 90° with respect to the rolling directionof the steel sheet, by a tensile test in accordance with the descriptionof JIS Z 2241 (2011) at a crosshead speed of 10 mm/min.

Excellent surface properties means that the length incidence of thescale defects per 100 coils is 0.8% or less. (The “scale defects” meansthat the scale remaining during descaling during the hot-rolling stepremains in the finish rolling step and the subsequent steps, and formsdefects that degrade the surface properties. Examples of the scaledefects include red scale and bite scale.)

The inventors of the present invention have conducted extensive studiesto obtain a steel sheet that has a TS of 340 MPa or more, excellentworkability, excellent YP planar anisotropy, and excellent surfaceproperties, and a method for producing the steel sheet, and have foundthe following.

By adding particular amounts or more of P and S having an excellentdescaling property, the surface properties of the steel sheet can beimproved. Moreover, the YP planar anisotropy can be decreased bycontrolling cold-rolling and the heating rate in the recrystallizationtemperature range during the heating process in annealing and bycontrolling the ferrite recrystallized texture. In addition, ductilitycan be improved by appropriately controlling the overaging temperatureand by controlling cementite to precipitate in the grain boundaries offerrite.

On the basis of the findings described above, it has become possible toproduce a steel sheet that has a TS of 340 MPa or more, excellentworkability, excellent YP planar anisotropy, and excellent surfaceproperties.

The present invention has been made on the basis of the above-describedfindings. In other words, the summary of the features according toexemplary embodiments of the present invention is as follows.

[1] A steel sheet having: a composition containing, in terms of mass %,C: 0.010% or more and 0.150% or less, Si: 0.20% or less, Mn: 1.00% orless, P: 0.100% or less, S: 0.0500% or less, Al: 0.001% or more and0.100% or less, N: 0.0100% or less, and the balance being Fe andunavoidable impurities, in which 0.002% [% P]+[% S]≤0.070% ([% M]denotes a content (mass %) of M element in steel) is satisfied; a steelstructure in which ferrite has an average crystal grain size of 5 μm ormore and 25 μm or less, 40% or more of cementite in terms of areafraction is precipitated in ferrite grain boundaries, and the ferritehas a texture in which an inverse intensity ratio of γ-fiber to α-fiberis 0.8 or more and 7.0 or less; and a tensile strength of 340 MPa ormore.

[2] The steel sheet described in [1], wherein the composition furthercontains, in terms of mass %, at least one element selected from: Ti:0.001% or more and 0.100% or less, Nb: 0.001% or more and 0.100% orless, V: 0.001% or more and 0.100% or less, B: 0.0001% or more and0.0050% or less, Cr: 0.01% or more and 1.00% or less, Mo: 0.01% or moreand 0.50% or less, Cu: 0.01% or more and 1.00% or less, Ni: 0.01% ormore and 1.00% or less, As: 0.001% or more and 0.500% or less, Sb:0.001% or more and 0.200% or less, Sn: 0.001% or more and 0.200% orless, Ta: 0.001% or more and 0.100% or less, Ca: 0.0001% or more and0.0200% or less, Mg: 0.0001% or more and 0.0200% or less, Zn: 0.001% ormore and 0.020% or less, Co: 0.001% or more and 0.020% or less, Zr:0.001% or more and 0.020% or less, and REM: 0.0001% or more and 0.0200%or less.

[3] A coated steel sheet including the steel sheet described in [1] or[2], and a coating layer on a surface of the steel sheet.

[4] A method for producing a hot-rolled steel sheet, the methodincluding heating a steel slab having the composition described in [1]or [2]; rough-rolling the heated steel slab; in subsequent finishrolling, hot-rolling the rough-rolled steel slab under conditions of afinish-rolling inlet temperature of 1020° C. or higher and 1180° C. orlower, a rolling reduction in a final pass of the finish rolling of 5%or more and 15% or less, a rolling reduction in a pass before the finalpass of 15% or more and 25% or less, and a finish-rolling deliverytemperature of 800° C. or higher and 1000° C. or lower; after the hotrolling, cooling the hot-rolled sheet to a coiling temperature at anaverage cooling rate of 5° C./s or more and 90° C./s or less; andcoiling the cooled sheet at a coiling temperature of 400° C. or higherand 800° C. or lower.

[5] A method for producing a cold-rolled full hard steel sheet, themethod including pickling a hot-rolled steel sheet obtained in themethod described in [4]; and performing cold-rolling at a rollingreduction of 55% or more, wherein, when a rolling reduction in a finalpass of cold rolling is assumed to be R_(F) and rolling reductions onestand, two stands, and three stands before the final pass arerespectively assumed to be R_(F-1), R_(F-2), and R_(F-3), the rollingreductions R_(F-1), R_(F-2), and R_(F-3) are each set to 10% or more and35% or less, a difference (|R_(F-1)−R_(F-2)|) between the rollingreduction one stand before the final pass and the rolling reduction twostands before the final pass is set to 10% or less, and a difference(|R_(F-2)−R_(F-3)|) between the rolling reduction two stands before thefinal pass and the rolling reduction three stands before the final passis set to 10% or less.

[6] A method for producing a steel sheet, the method including heating,in a continuous annealing furnace, a cold-rolled full hard steel sheet,which is obtained in the method described in [5], while setting a dewpoint to −40° C. or lower in a temperature range of 600° C. or higherand setting an average heating rate to 50° C./s or less in a temperaturerange of 450° C. to T1 temperature−10° C.; holding the sheet in atemperature range of the T1 temperature or higher and a T2 temperatureor lower; cooling the heated sheet to an overaging temperature; and thenperforming an overaging process at a temperature of 300° C. or higherand 550° C. or lower, where:

T1 temperature (° C.)=735+29×[% Si]−21×[% Mn]+17×[% Cr]

T2 temperature (° C.)=960−203×[% C]^(1/2)+45×[% Si]−30×[% Mn]+150×[%Al]−20×[% Cu]+11×[% Cr]+350×[% Ti]+104×[% V]

where in the formulae above, [% X] denotes a content (mass %) of acomponent element X in the steel sheet, and 0 is indicated when theelement is not contained.

[7] The method for producing a steel sheet described in [6], in whichthe cooling to the overaging temperature is performed by water cooling.

[8] A method for producing a steel sheet, the method including heatingand holding a cold-rolled full hard steel sheet, which is obtained inthe method described in [5], to and in a temperature range of 600° C. orhigher and 750° C. or lower in a box annealing furnace while setting adew point to −40° C. or lower in a temperature range of 600° C. orhigher.

[9] A method for producing a coated steel sheet, the method includingcoating the steel sheet obtained in the method described in any one of[6] to [8].

A steel sheet and a coated steel sheet obtained according to embodimentsby the present invention have a TS of 340 MPa or more, excellentworkability, excellent YP planar anisotropy, and excellent surfaceproperties. When the steel sheet and the coated steel sheet according toembodiments of the present invention are applied to, for example,automobile structural elements, fuel efficiency can be improved throughcar body weight reduction, and thus the present invention offersconsiderable industrial advantages.

Furthermore, the method for producing a hot-rolled steel sheet and themethod for producing a cold-rolled full hard steel sheet according to anembodiment of the present invention serve as the methods for producingintermediate products for obtaining the steel sheet and the coated steelsheet with excellent properties described above and contribute toimproving the properties of the steel sheet and the coated steel sheetdescribed above.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The embodiments of the present invention will now be described. Itshould be understood that the present invention is not limited to thefollowing embodiments.

The present invention provides a steel sheet, a coated steel sheet, amethod for producing a hot-rolled steel sheet, a method for producing acold-rolled full hard steel sheet, a method for producing a steel sheet,and a method for producing a coated steel sheet. First, how these relateto one another is described.

A steel sheet according to embodiments of the present invention alsoserves as an intermediate product for obtaining a coated steel sheet ofthe present invention. A coated steel sheet according to an embodimentof the present invention is prepared from a steel material, such as aslab, and obtained by going through the processes of producing ahot-rolled steel sheet, a cold-rolled full hard steel sheet, and a steelsheet. The steel sheet according to embodiments of the present inventionis the steel sheet used in the above-described process.

The method for producing a hot-rolled steel sheet according toembodiments of the present invention is the method that covers up toobtaining a hot-rolled steel sheet in the process described above.

The method for producing a cold-rolled full hard steel sheet accordingto embodiments of the present invention is the method that covers up toobtaining a cold-rolled full hard steel sheet from a hot-rolled steelsheet in the process described above.

The method for producing a steel sheet according to embodiments of thepresent invention is the method that covers up to obtaining a steelsheet from a cold-rolled full hard steel sheet in the process describedabove.

The method for producing a coated steel sheet according to embodimentsof the present invention is the method that covers up to obtaining acoated steel sheet from a steel sheet in the process described above.

Since such a relationship exists, the compositions of the hot-rolledsteel sheet, the cold-rolled full hard steel sheet, the steel sheet, andthe coated steel sheet are common, and the steel structures of the steelsheet and the coated steel sheet are common. In the description below,the common features, the steel sheet, the coated steel sheet, and theproduction methods therefor are described in that order.

<Composition>

The steel sheet etc., according to embodiments of the present inventionhave a composition containing, in terms of mass %, C: 0.010% or more and0.150% or less, Si: 0.20% or less, Mn: 1.00% or less, P: 0.100% or less,S: 0.0500% or less, Al: 0.001% or more and 0.100% or less, N: 0.0100% orless, and the balance being Fe and unavoidable impurities, in which0.002%≤[% P]+[% S]≤0.070% is satisfied ([% M] denotes a content (mass %)of an M element in the steel).

The composition may further contain, in terms of mass %, at least oneelement selected from Ti: 0.001% or more and 0.100% or less, Nb: 0.001%or more and 0.100% or less, V: 0.001% or more and 0.100% or less, B:0.0001% or more and 0.0050% or less, Cr: 0.01% or more and 1.00% orless, Mo: 0.01% or more and 0.50% or less, Cu: 0.01% or more and 1.00%or less, Ni: 0.01% or more and 1.00% or less, As: 0.001% or more and0.500% or less, Sb: 0.001% or more and 0.200% or less, Sn: 0.001% ormore and 0.200% or less, Ta: 0.001% or more and 0.100% or less, Ca:0.0001% or more and 0.0200% or less, Mg: 0.0001% or more and 0.0200% orless, Zn: 0.001% or more and 0.020% or less, Co: 0.001% or more and0.020% or less, Zr: 0.001% or more and 0.020% or less, and REM: 0.0001%or more and 0.0200% or less.

The individual components will now be described. In the descriptionbelow, “%” that indicates the content of the component means “mass %”.

C: 0.010% or more and 0.150% or less

C either forms cementite in the steel or exists in a solid solutionstate. At a C content less than 0.010%, precipitation driving force ofsolid solution C is decreased, cementite precipitation is inhibited, andthe strength is degraded. Thus, the C content is to be 0.010% or more.More preferably, the C content is 0.030% or more. At a C contentexceeding 0.150%, the amount of cementite is increased, the number ofvoid-generation sites at the cementite-ferrite interfaces increasesduring working, and the elongation of the steel sheet is degraded. Thus,the C content is to be 0.150% or less. The C content is preferably0.130% or less and more preferably 0.120% or less.

Si: 0.20% or less

Si is an element that suppresses generation of cementite, and suppressescementite formation from C. Thus, at a Si content exceeding 0.20%, thesites where cementite precipitates are no longer controlled, andcementite is likely to emerge inside ferrite grains. Meanwhile, at a Sicontent exceeding 0.20%, the descaling property and the picklingproperty (descalability) are degraded, and the surface properties aredeteriorated. Thus, the Si content is to be 0.20% or less. The Sicontent is preferably 0.10% or less and more preferably 0.05% or less.In an embodiment of the present invention, the Si content is usually0.01% or more.

Mn: 1.00% or less

Manganese (Mn) does not form a compound with C, but Mn and C attracteach other in the steel, and diffusion of C is suppressed. Thus,incorporation of Mn suppresses generation of cementite in grainboundaries, and deteriorates workability. Thus, in the presentinvention, the content of Mn is preferably reduced as with Si. Thus, theMn content is to be 1.00% or less. The Mn content is preferably 0.80% orless and more preferably 0.70% or less. In the present invention, the Mncontent is usually 0.10% or more.

P: 0.100% or less

Phosphorus (P) segregates in ferrite grain boundaries, suppressesprecipitation of cementite in the ferrite grain boundaries, and therebydeteriorates the workability. Thus, the P content is to be 0.100% orless. The P content is preferably 0.060% or less, more preferably 0.050%or less, and yet more preferably 0.040% or less. In the presentinvention, the P content is usually 0.001% or more and preferably 0.003%or more.

S: 0.0500% or less

Sulfur (S) is an element that bonds with Mn to form MnS. When the Scontent is large, a large amount of MnS is generated and obstructs graingrowth of ferrite grains, the ferrite grains become fine as a result,and the workability is deteriorated. Thus, in an embodiment of thepresent invention, the S content is to be 0.0500% or less. The S contentis preferably 0.0300% or less and more preferably 0.0200% or less. Inthe present invention, the S content is usually 0.0010% or more.

Al: 0.001% or more and 0.100% or less

Aluminum (Al) is an important element in an embodiment of the presentinvention. Although Al itself does not form carbides, Al promotesremoving C from the inside of the ferrite grains and promotes formationof cementite in the grain boundaries. In order to obtain these effects,the Al content needs to be at least 0.001%. Preferably, the Al contentis 0.005% or more. As a result, workability is improved. However, at anAl content exceeding 0.100%, Al bonds with fine AlN and oxygen (O),which is an unavoidable impurity, to form fine oxides and makes ferritegrains finer, thereby deteriorating the workability. Thus, the Alcontent is to be 0.100% or less. Thus, the Al content is to be in therange of 0.001% or more and 0.100% or less. The lower limit of the Alcontent is preferably 0.005% or more, and the upper limit of the Alcontent is preferably 0.100% or less and more preferably 0.070% or less.

N: 0.0100% or less

Nitrogen (N) bonds with Al to form AlN. When B is added, N forms BN.When the N content is large, a large amount of nitrides occur andobstruct grain growth of ferrite grains, the ferrite grains become fineas a result, and the workability is deteriorated. Thus, in an embodimentof the present invention, the N content is set to be 0.0100% or less.The N content is preferably 0.0080% or less and more preferably 0.0070%or less. In the present invention, the N content is usually 0.0010% ormore.

The steel sheet etc., of the present invention may contain, in additionto the composition described above, in terms of mass %, at least oneelement selected from Ti: 0.001% or more and 0.100% or less, Nb: 0.001%or more and 0.100% or less, V: 0.001% or more and 0.100% or less, B:0.0001% or more and 0.0050% or less, Cr: 0.01% or more and 1.00% orless, Mo: 0.01% or more and 0.50% or less, Cu: 0.01% or more and 1.00%or less, Ni: 0.01% or more and 1.00% or less, As: 0.001% or more and0.500% or less, Sb: 0.001% or more and 0.200% or less, Sn: 0.001% ormore and 0.200% or less, Ta: 0.001% or more and 0.100% or less, Ca:0.0001% or more and 0.0200% or less, Mg: 0.0001% or more and 0.0200% orless, Zn: 0.001% or more and 0.020% or less, Co: 0.001% or more and0.020% or less, Zr: 0.001% or more and 0.020% or less, and REM: 0.0001%or more and 0.0200% or less. These elements are preferably containedalone or in combination.

Titanium (Ti) is an element effective for fixing N, which induces agingdegradation, by forming TiN. This effect is obtained by setting the Ticontent to 0.001% or more. Meanwhile, at a Ti content exceeding 0.100%,TiC is excessively generated in the hot rolling stage, the amount ofcarbides generated inside the ferrite grains increases, and, thus,deformation of ferrite is suppressed and the workability is degraded.Thus, if Ti is to be added, the Ti content is set within a range of0.001% or more and 0.100% or less, and the lower limit is preferably0.005% or more. The upper limit is preferably 0.050% or less.

Niobium (Nb) forms fine precipitates during hot-rolling or annealing,and increases the strength. Niobium also reduces the size of grainsduring hot-rolling, and accelerates recrystallization of ferrite, whichcontributes to decreasing the YP planar anisotropy, during cold-rollingand the subsequent annealing. In order to obtain these effects, the Nbcontent needs to be 0.001% or more. Meanwhile, at a Nb content exceeding0.100%, composite precipitates, such as Nb—(C, N) are, excessivelygenerated, the amount of carbides generated inside the ferrite grainsincreases, and, thus, deformation of ferrite is suppressed and theworkability is degraded. Thus, if Nb is to be added, the Nb content isset within a range of 0.001% or more and 0.100% or less. The lower limitof the Nb content is preferably 0.005% or more. The upper limit of theNb content is preferably 0.050% or less.

Vanadium (V) can increase the strength of steel by forming carbides,nitrides, or carbonitrides. In order to obtain this effect, the Vcontent needs to be 0.001% or more. Meanwhile, when the V content isexcessively large, TiC is excessively generated in the hot rollingstage, the amount of carbides generated inside the ferrite grainsincreases, and, thus, deformation of ferrite is suppressed and theworkability is degraded. Thus, if V is to be added, the V content is setwithin a range of 0.001% or more and 0.100% or less. The lower limit ofthe V content is preferably 0.005% or more and more preferably 0.010% ormore. The upper limit of the V content is preferably 0.080% or less andmore preferably 0.070% or less.

Boron (B) bonds with N to form BN and can suppress precipitation of fineAlN. Since BN precipitates by using MnS as a nucleus, the amount of fineMnS can also be decreased, and as a result, ferrite grain growth can bepromoted. The effect of adding B is obtained by setting the B content to0.0001% or more. However, at a B content exceeding 0.0050%, excess Bsegregates in the ferrite grain boundaries, and solid solution Bsuppresses precipitation of cementite in the ferrite grain boundaries,thereby deteriorating the workability. Thus, the B content is to be0.0050% or less.

Chromium (Cr) is a solid solution strengthening element effective forstrengthening the steel, and, in order to obtain this effect, the Crcontent needs to be 0.01% or more. Meanwhile, at a Cr content exceeding1.00%, enhancement of the effect is rarely achieved, coatability isinhibited, and bare spot defects are generated. Thus, if Cr is to beadded, the Cr content is set within a range of 0.01% or more and 1.00%or less.

Molybdenum (Mo) is an element effective for strengthening the steelwithout degrading chemical conversion treatability and coatability. Thiseffect is obtained by setting the Mo content to 0.01% or more. However,at a Mo content exceeding 0.50%, Mo forms coarse carbides and degradesductility, the amount of inclusions and the like is increased, defectsand the like occur in the surface or in the inside, and the ductility issignificantly degraded. Thus, the Mo content is set within a range of0.01% or more and 0.50% or less.

Copper (Cu) is a solid solution strengthening element effective forstrengthening the steel, and, in order to obtain this effect, the Cucontent needs to be 0.01% or more. However, at a Cu content exceeding1.00%, the surface layer may crack during hot-rolling, the amount ofinclusions and the like increases, defects and the like are therebyformed in the surface or in the inside, and the ductility issignificantly degraded. Thus, if Cu is to be added, the Cu content isset within a range of 0.01% or more and 1.00% or less.

Nickel (Ni) contributes to increasing the strength by solid solutionstrengthening. In order to obtain this effect, the Ni content needs tobe 0.01% or more. However, when an excessive amount of Ni is added, thesurface layer may crack during hot-rolling, the amount of inclusions andthe like increases, the defects and the like are thereby formed in thesurface or in the inside, and the ductility is significantly degraded.Thus, if Ni is to be added, the Ni content is set within a range of0.01% or more and 1.00% or less. The Ni content is preferably 0.50% orless.

Arsenic (As) is an element effective for improving corrosion resistance.In order to obtain this effect, the As content needs to be 0.001% ormore. However, if As is added excessively, red shortness is accelerated,the amount of inclusions and the like increases, the defects and thelike are thereby formed in the surface or in the inside, and theductility is significantly degraded. Thus, if As is to be added, the Ascontent is set within a range of 0.001% or more and 0.500% or less.

Antimony (Sb) and tin (Sn) are added as needed from the viewpoint ofsuppressing decarburization that occurs due to nitriding or oxidizing ofthe steel sheet surface in a region that spans about several tenmicrometers from the steel sheet surface in the sheet thicknessdirection. Suppressing nitridation and oxidation is effective forimproving the surface properties. In order to obtain these effects, thecontent needs to be 0.001% or more for Sb and for Sn. Meanwhile, if anyof these elements is contained in an amount exceeding 0.200%, toughnessis degraded. Thus, if Sb and Sn are to be added, the content is setwithin a range of 0.001% or more and 0.200% or less for each of theelements.

Tantalum (Ta) contributes to increasing the strength by forming alloycarbides and alloy carbonitrides as with Ti and Nb. In addition, Ta isconsidered to have an effect of partly dissolving in Nb carbides and/orNb carbonitrides to form composite precipitates such as (Nb, Ta)(C, N)so as to significantly suppress coarsening of precipitates and stabilizethe contribution to improving the strength of the steel sheet byprecipitation strengthening. Thus, Ta is preferably contained. Here, theeffect of stabilizing the precipitates described above is obtained bysetting the Ta content to 0.001% or more; however, when Ta isexcessively added, the precipitate stabilizing effect is saturated, theamount of inclusions and the like increases, the defects and the likeare thereby formed in the surface or in the inside, and the ductility issignificantly degraded. Thus, if Ta is to be added, the Ta content isset within a range of 0.001% or more and 0.100% or less.

Calcium (Ca) and magnesium (Mg) are elements used for deoxidization, andalso are elements that are effective for making sulfides spherical andalleviating adverse effects of sulfides on ductility, in particular,local ductility. In order to obtain these effects, at least one of theseelements needs to be contained in an amount of 0.0001% or more. However,if the amount of at least one element selected from Ca and Mg exceeds0.0200%, the amount of inclusions and the like increases, the defectsand the like are thereby formed in the surface or in the inside, and theductility is significantly degraded. Thus, if Ca and Mg are to be added,the content is set within a range of 0.0001% or more and 0.0200% or lessfor each of the elements.

Zinc (Zn), cobalt (Co), and zirconium (Zr) are all an element effectivefor improving corrosion resistance. In order to obtain this effect, atleast one of these elements needs to be contained in an amount of 0.001%or more. However, when the content of at least one of Zn, Co, and Zrexceeds 0.020%, that element segregates in the grain boundaries andobstructs precipitation of cementite in the grain boundaries. Thus, ifZn, Co, and Zr are to be added, the content is set within a range of0.001% or more and 0.020% or less for each of the elements.

A rare earth metal (REM) is an element effective for improving corrosionresistance. In order to obtain this effect, the REM content needs to be0.0001% or more. However, when the REM content exceeds 0.0200%, REMsegregates in the grain boundaries and obstructs precipitation ofcementite in the grain boundaries. Thus, if REM is to be added, the REMcontent is set within a range of 0.0001% or more and 0.0200% or less.

The balance other than the above-described components is Fe andunavoidable impurities. For optional components (optional elements)described above, if their contents are less than the lower limits, theeffects of the present invention are not impaired; thus, when theseoptional elements are contained in amounts less than the lower limits,these optional elements are deemed to be contained as unavoidableimpurities.

Although the components in the composition of the steel sheet aredescribed above, in order to obtain the anticipated effects of thepresent invention, it is not sufficient to adjust the composition to bewithin the ranges described above, and it is important to appropriatelycontrol the total of the P content and the S content.

That is, the composition needs to satisfy 0.002%≤[% P]+[% S]≤0.070%(where [% M] denotes the content (mass %) of the M element in thesteel).

0.002%≤[% P]+[% S]≤0.070%

This is an extremely important composition prescription in theembodiments of present invention. Phosphorus (P) and sulfur (S) lowerthe melting point of the subscale composition since they bring acompositional change in the subscale (FeO/FeS/P₂O₅) generated at thescale-base iron interface as the scale is generated in the hot-rollingstage; and thus P and S can improve the descaling property and thepickling property (descalability) in the pickling step. In order toobtain these effects, the total of the P content and the S content needsto be 0.002% or more. Meanwhile, when the total of the P content and theS content exceeds 0.070%, enhancement of the effect is rarely obtained,and, as mentioned above, the workability is degraded. Thus, the total ofthe P content and the S content is set to be 0.002% or more and 0.070%or less. The lower limit is preferably 0.003%≤[% P]+[% S]. The upperlimit is preferably [% P]+[% S]≤0.060% and more preferably [% P]+[%S]≤0.055%.

<Steel Structure>

In the steel structure of the steel sheet and the like according toembodiments of the present invention, ferrite has an average crystalgrain size of 5 μm or more and 25 μm or less, 40% or more of cementitein terms of area fraction precipitates in ferrite grain boundaries, andthe ferrite has a texture in which an inverse intensity ratio of γ-fiberto α-fiber is 0.8 or more and 7.0 or less.

Average crystal grain size of ferrite: 5 μm or more and 25 μm or less

When the average crystal grain size of ferrite is less than 5 μm, theyield stress increases, the elongation decreases, and thus theworkability is degraded. Thus, the average crystal grain size is to be 5μm or more. The average crystal grain size of ferrite is preferably 8 μmor more. However, when the ferrite grain size (average crystal grainsize of ferrite) exceeds 25 μm, significant irregularities called orangepeel form on the surface during working, and this degrades theworkability, deteriorates the appearance quality, and also degrades thestrength. Thus, the ferrite grain size is to be 25 μm or less. From theviewpoint of obtaining anticipated properties, the ferrite grain size ispreferably 23 μm or less.

The average crystal grain size of ferrite is calculated as follows. Thatis, the observation position is set to the position of ¼ of the sheetthickness from the surface at a section taken in parallel to the rollingdirection, the steel sheet is observed with an optical microscope at amagnification of about 50, and, by using Adobe Photoshop, the total areaof the ferrite grains within the observation view area is divided by thenumber of ferrite grains so as to calculate the average area of theferrite. The calculated average area is raised to the power of ½, andthe result is assumed to be the average crystal grain size of ferrite.

Cementite: 40% or more is precipitated in ferrite grain boundaries

The position where cementite precipitates is important in an embodimentof the present invention. Having cementite in the ferrite grainboundaries decreases the amount of fine cementite present inside thegrains, and deformation of ferrite grains can be promoted. When theamount of cementite present in the ferrite grain boundaries is less than40% of all cementite, fine cementite inside the ferrite grainssuppresses deformation of ferrite and degrades the workability. Thus,40% or more of the sites where cementite has precipitated need to be inthe ferrite grain boundaries. Preferably, the percentage is 45% or more.

The amount of the cementite that has precipitated in the ferrite grainboundaries can be determined from the section structure as follows. Theobservation position is set to the position at ¼ thickness in asheet-thickness section taken parallel to the rolling direction, and thesteel structure is observed at that position. After the section ismirror-polished and cementite is exposed by using a picral corrosivesolution, the cementite is observed with an optical microscope (500magnification). Here, the ratio of the area of cementite that exists inthe ferrite grain boundaries to the area of all cementite is assumed tobe the ratio of the cementite that exists in the grain boundaries. Thephrase “exists in grain boundaries” means cementite appears to beentirely inside the grain boundaries, appears to be partly inside thegrain boundaries, or appears to contact the grain boundaries in amicroscope image obtained as above.

Inverse intensity ratio of γ-fiber to the α-fiber in ferrite texture:0.8 or more and 7.0 or less

α-Fiber is a fibrous texture whose <110> axis is parallel to the rollingdirection, and γ-fiber is a fibrous texture whose <111> axis is parallelto the normal direction of the rolled surface. A body-centered cubicmetal is characterized in that α-fiber and γ-fibers strongly develop dueto rolling deformation, and the textures that belong to these fibers areformed even if annealing is conducted.

In an embodiment of the present invention, when the inverse intensityratio of γ-fiber to the α-fiber in the ferrite texture exceeds 7.0, thetexture orients in a particular direction of the steel sheet, and theplanar anisotropy of mechanical properties, in particular, the planaranisotropy of the YP, is increased. Meanwhile, even when the inverseintensity ratio of γ-fiber to the α-fiber in the ferrite texture is lessthan 0.8, the planar anisotropy of mechanical properties, in particular,the planar anisotropy of the YP, is also increased. Thus, the inverseintensity ratio of γ-fiber to the α-fiber in the ferrite texture is tobe 0.8 or more and 7.0 or less, and the lower limit of the intensityratio is preferably 2.0 or more and more preferably 2.5 or more. Theupper limit of the intensity ratio is preferably 6.5 or less.

In the present invention, the inverse intensity ratio of γ-fiber to theα-fiber in the ferrite texture can be obtained as follows. After asheet-thickness section (L section) parallel to the rolling direction ofthe steel sheet is wet-polished and buff-polished with a colloidalsilica solution so as to make the surface smooth and flat, the sectionis corroded with a 0.1 vol. % nital so as to minimize irregularities onthe sample surface and completely remove the work-deformed layer. Next,at a position ¼ of the sheet thickness (the position at a depth of ¼ ofthe sheet thickness from the steel sheet surface), crystal orientationis measured by SEM-EBSD (electron back-scatter diffraction), and theobtained data is analyzed by using OIM analysis available from AMETEKEDAX Company so as to calculate the inverse intensity ratio of γ-fiberto the α-fiber in ferrite.

The steel structure required to obtain the effects according toembodiments of the present invention is as described above; however,typically, in the present invention, 50% or more of ferrite and 5% ormore and 40% or less of cementite are contained in terms of areafraction. Other phases may also be contained as long as the effects arenot adversely affected.

<Steel Sheet>

The composition and the steel structure of the steel sheet are asdescribed above. The thickness of the steel sheet is not particularlylimited but is typically 0.3 mm or more and 2.8 mm or less.

<Coated Steel Sheet>

A coated steel sheet of the present invention is constituted by thesteel sheet according to embodiments of the present invention and acoating layer on the steel sheet. The type of the coating layer is notparticularly limited, and may be, for example, a hot-dip coating layeror an electrocoating layer. The coating layer may be an alloyed coatinglayer. The coating layer is preferably a zinc coating layer. The zinccoating layer may contain Al and Mg. A hot-dip zinc-aluminum-magnesiumalloy coating (Zn—Al—Mg coating layer) is also preferable. In this case,the Al content is preferably 1 mass % or more and 22 mass % or less, theMg content is preferably 0.1 mass % or more and 10 mass % or less, andthe balance is preferably Zn. In the case of the Zn—Al—Mg coating layer,a total of 1 mass % or less of at least one element selected from Si,Ni, Ce, and La may be contained in addition to Zn, Al, and Mg. Thecoating metal is not particularly limited, and Al coating and the likemay be used in addition to the Zn coating described above. The coatingmetal is not particularly limited, and Al coating and the like may beused in addition to the Zn coating described above.

The composition of the coating layer is also not particularly limitedand may be any typical composition. For example, in the case of agalvanizing layer or a galvannealing layer, typically, the compositioncontains Fe: 20 mass % or less and Al: 0.001 mass % or more and 1.0 mass% or less, a total of 0 mass % or more and 3.5 mass % or less of one ormore elements selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu,Li, Ti, Be, Bi, and REM, and the balance being Zn and unavoidableimpurities. In the present invention, a galvanizing layer having acoating weight of 20 to 80 g/m² per side, or a galvannealing layerobtained by alloying this galvanizing layer is preferably provided. Whenthe coating layer is a galvanizing layer, the Fe content in the coatinglayer is less than 7 mass %, and when the coating layer is agalvannealing layer, the Fe content in the coating layer is 7 to 20 mass%.

<Method for Producing Hot-Rolled Steel Sheet>

A method for producing a hot-rolled steel sheet according to embodimentsof the present invention includes heating a steel slab having theabove-described composition; hot-rolling the heated slab underconditions of a rolling reduction 5% or more and 15% or less in thefinal pass of the finish rolling, a rolling reduction of 15% or more and25% or less in a pass before the final pass, a finish-rolling inlettemperature of 1020° C. or higher and 1180° C. or lower, and afinish-rolling delivery temperature of 800° C. or higher and 1000° C. orlower; after the hot-rolling, cooling to a coiling temperature at anaverage cooling rate of 5° C./s or more and 90° C./s or less; andcoiling the cooled sheet at a temperature of 400° C. or higher and 800°C. or lower. In the description below, the temperature is a steel sheetsurface temperature unless otherwise noted. The steel sheet surfacetemperature can be measured with a radiation thermometer or the like.

In the present invention, the method for melting the steel material(steel slab) is not particularly limited, and any known melting methodsuch as one using a converter or an electric furnace is suitable. Thecasting method is also not particularly limited, but a continuouscasting method is preferable. The steel slab (slab) is preferablyproduced by a continuous casting method to prevent macrosegregation, butcan be produced by an ingot-making method, a thin-slab casting method,or the like. In addition to a conventional method that involves coolingthe produced steel slab to room temperature and then re-heating thecooled steel slab, an energy-saving process, such as hot direct rolling,that involves directly charging a hot steel slab into a heating furnacewithout performing cooling to room temperature or rolling the steel slabimmediately after very short recuperation can be employed without anyissues. Moreover, the slab is formed into a sheet bar by rough-rollingunder standard conditions; however, if the heating temperature is setrelatively low, the sheet bar is preferably heated with a bar heater orthe like before finish rolling in order to prevent troubles that occurduring hot-rolling. In hot-rolling the slab, the slab may be re-heatedin a heating furnace and then hot-rolled, or may be heated in a heatingfurnace at 1250° C. or higher for a short period of time and thenhot-rolled.

The steel material (slab) obtained as such is subjected to hot-rolling.In this hot-rolling, only rough rolling and finish rolling may beperformed, or only finish rolling may be performed without roughrolling. In either case, the rolling reduction in the final pass of thefinish rolling, the rolling reduction in the pass immediately before thefinal pass, the finish-rolling inlet temperature, and the finish-rollingdelivery temperature are important.

Rolling reduction in final pass of finish rolling: 5% or more and 15% orless

Rolling reduction in pass before final pass: 15% or more and 25% or less

In the present invention, when the rolling reduction in the pass beforethe final pass is set to be equal to or more than the rolling reductionin the final pass, the average crystal grain size of ferrite, thedispersion state of cementite, and the texture can be appropriatelycontrolled. Thus, this feature is extremely important. When the rollingreduction in the final pass of the finish rolling is less than 5%, theferrite crystal grains coarsen during hot-rolling, the crystal grainsthereby coarsen in cold-rolling and subsequent annealing, and thus, thestrength is degraded. Moreover, ferrite nucleation and growth occursfrom very coarse austenite grains, and thus a so-called duplex-grainedstructure in which the generated ferrite grains vary in size is created.As a result, grains of a particular orientation grow duringrecrystallization annealing, resulting in an increase in YP planaranisotropy. Meanwhile, when the rolling reduction in the final passexceeds 15%, the ferrite crystal grains become finer during hot-rolling,the crystal grains become finer in cold-rolling and subsequentannealing, and thus, the strength is increased. Moreover, as a result ofthe increase in the percentage of the cementite precipitated inside theferrite grains, the amount of fine cementite inside the ferrite grainsincreases, deformation of ferrite is suppressed, and the workability isdegraded. Thus, the rolling reduction in the final pass of the finishrolling is set to be 5% or more and 15% or less.

When the rolling reduction in the pass before the final pass is lessthan 15%, a duplex-grained structure in which the generated ferritegrains vary in size is created despite rolling of the very coarseaustenite grains in the final pass, and, as a result, grains of aparticular orientation grow during recrystallization annealing,resulting in an increase in YP planar anisotropy. Meanwhile, when therolling reduction in the pass before the final pass exceeds 25%, theferrite crystal grains become finer during hot-rolling, the crystalgrains become finer in cold-rolling and subsequent annealing, and thus,strength is increased. Moreover, as a result of the increase in thepercentage of the cementite precipitated inside the ferrite grains, theamount of fine cementite within the ferrite grains increases,deformation of ferrite is suppressed, and the workability is degraded.Thus, the rolling reduction in the pass before the final pass in thefinish annealing is set to be 15% or more and 25% or less.

Finish-rolling inlet temperature: 1020° C. or higher and 1180° C. orlower

The steel slab after heating is hot-rolled through rough rolling andfinish rolling so as to form a hot-rolled steel sheet. During thisprocess, when the finish-rolling inlet temperature exceeds 1180° C., theamount of oxides (scale) generated increases rapidly, the interfacebetween the base iron and oxides is roughened, the scale separabilityduring descaling or pickling is degraded, and thus the surface qualityafter annealing is deteriorated. Moreover, if unseparated hot-rolledscale remains in some parts after pickling, ductility is adverselyaffected. Meanwhile, at a finish-rolling inlet temperature lower than1020° C., the finish-rolling temperature after finish-rolling decreases,the rolling load during hot-rolling increases, and the rolling workloadincreases. Moreover, the rolling reduction while austenite is in anun-recrystallized state is increased, control of the texture afterrecrystallization annealing becomes difficult, and significant planaranisotropy is generated in the final product, thereby degrading theuniformity and stability of the materials, and ductility itself. Thus,the finish-rolling inlet temperature of hot-rolling needs to be 1020° C.or higher and 1180° C. or lower. Preferably, the temperature is 1020° C.or higher and 1160° C. or lower.

Finish-rolling delivery temperature: 800° C. or higher and 1000° C. orlower

The steel slab after heating is hot-rolled through rough rolling andfinish rolling so as to form a hot-rolled steel sheet. During thisprocess, when the finish-rolling delivery temperature exceeds 1000° C.,the amount of oxides (scale) generated increases rapidly, the interfacebetween the base iron and oxides is roughened, the scale separabilityduring descaling or pickling is degraded, and thus the surface qualityafter annealing is deteriorated. Moreover, if unseparated hot-rolledscale remains in some parts after pickling, ductility is adverselyaffected. In addition, the crystal grains excessively coarsen, and thesurface of a press product may become rough during working. Meanwhile,when the finish-rolling delivery temperature is lower than 800° C., therolling load increases, the rolling workload increases, the rollingreduction while austenite is in an un-recrystallized state increases, anabnormal texture develops, and significant planar anisotropy isgenerated in the final product. Thus, the uniformity and stability ofthe materials, and the ductility itself are degraded. At afinish-rolling delivery temperature lower than 800° C., formation of thebanded structure increases, and the banded structure remains afterannealing and degrades the workability. Thus, the finish-rollingdelivery temperature of hot-rolling needs to be 800° C. or higher and1000° C. or lower. The lower limit of the finish-rolling deliverytemperature is preferably 820° C. or higher. The upper limit of thefinish-rolling delivery temperature is preferably 950° C. or lower.

As mentioned above, in this hot-rolling, only rough rolling and finishrolling may be performed, or only finish rolling may be performedwithout rough rolling.

Average cooling rate from after finish-rolling to coiling temperature:5° C./s or more and 90° C./s or less

By appropriately controlling the average cooling rate from afterfinish-rolling to the coiling temperature, the crystal grains of thephases in the hot-rolled steel sheet can be made finer, and, after thesubsequent cold rolling and annealing, cumulation of the texture can beincreased in the {111}//ND direction. Here, if the average cooling ratefrom after finish-rolling to the coiling temperature exceeds 90° C./s,the shape of the sheet is significantly degraded, and problems may arisein the subsequent cold-rolling or annealing (heating and cooling processafter cold-rolling). If the average cooling rate is less than 5° C./s,coarse carbides are formed during hot-rolling, and remain even afterannealing, which increases the amount of carbides precipitated insidethe ferrite grains and thereby degrades workability. Thus, the averagecooling rate from after the finish-rolling to the coiling temperature isset to be 5° C./s or more and 90° C./s or less, and the lower limit ofthe average cooling rate is preferably 7° C./s or more and morepreferably 9° C./s or more. The upper limit of the average cooling rateis preferably 60° C./s or less and more preferably 50° C./s or less.

Coiling temperature: 400° C. or higher and 800° C. or lower

When the coiling temperature exceeds 800° C., ferrite grains becomecoarse, diffusion of C into ferrite grain boundaries does not happensoon enough during overaging, and the workability is deteriorated.Meanwhile, at a coiling temperature lower than 400° C. afterhot-rolling, precipitation of nitrides in the hot-rolled sheet issuppressed, and the nitrides form fine precipitates during annealingafter cold-rolling, and thereby suppress ferrite grain growth. Thus, thecoiling temperature after hot-rolling needs to be 400° C. or higher and800° C. or lower. The lower limit of the coiling temperature ispreferably 500° C. or higher and more preferably 540° C. or higher. Theupper limit of the coiling temperature is preferably 750° C. or lowerand more preferably 750° C. or lower.

During hot-rolling, rough-rolled sheets may be joined with each otherand finish-rolling may be conducted continuously. Moreover, therough-rolled sheet may be temporarily coiled. Furthermore, in order todecrease the rolling load during hot-rolling, part or the entirety ofthe finish-rolling may be lubricated. Performing lubricated rolling isalso effective from the viewpoints of uniformity of the steel sheetshape and uniformity of the material. The coefficient of friction duringlubricated rolling is preferably in the range of 0.10 or more and 0.25or less.

After the coiling, the steel sheet is cooled by air cooling or the likeand used to produce the cold-rolled full hard steel sheet describedbelow. When the hot-rolled steel sheet is treated as the subject of thetrade as an intermediate product, typically, the hot-rolled steel sheetthat is coiled and then cooled is traded.

<Method for Producing Cold-Rolled Full Hard Steel Sheet>

A method for producing a cold-rolled full hard steel sheet according toan embodiment of the present invention includes pickling the hot-rolledsteel sheet described above; and, cold-rolling the pickled sheet at arolling reduction of 55% or more, wherein, when a rolling reduction in afinal pass of cold rolling is assumed to be R_(F) and rolling reductionsone stand, two stands, and three stands before the final pass arerespectively assumed to be R_(F-1), R_(F-2), and R_(F-3) the rollingreductions R_(F-1), R_(F-2), and R_(F-3) are each set to 10% or more and25% or less, a difference (|R_(F-1)−R_(F-2)|) between the rollingreduction one stand before the final pass and the rolling reduction twostands before the final pass is set to 10% or less, and a difference(|R_(F-2)−R_(F-3)|) between the rolling reduction two stands before thefinal pass and the rolling reduction three stands before the final passis set to 10% or less.

Pickling can remove oxides on the steel sheet surface, and thus isextremely important for ensuring excellent chemical conversiontreatability and coating quality of the final products, such as steelsheets and coated steel sheets. Pickling may be performed once, or infractions several times.

Rolling reduction (rolling reduction ratio) in cold-rolling step: 55% ormore

When the rolling reduction in cold-rolling is less than 55%, cementitethat has precipitated in the ferrite grain boundaries in the hot-rolledsheet (hot-rolled steel sheet) remains as coarse even aftercold-rolling, and is likely to remain as cementite inside the ferritegrains in the cold-rolled annealed sheet (corresponding to the steelsheet), thereby resulting in degraded workability. Cold-rolling causesthe α-fiber and the γ-fiber to develop and thereby increases the amountof ferrite having the α-fiber and the γ-fiber, in particular, ferritehaving the γ-fiber, in a structure after annealing, and, thus, the YPplanar anisotropy can be decreased. Thus, the lower limit of the rollingreduction for cold-rolling is set to be 55%. The upper limit of therolling reduction is not particularly limited, but, from the industrialviewpoint, is about 95%.

Rolling reduction one stand before final pass of cold-rolling, androlling reductions two stands and three stands before final pass: each10% or more and 35% or less

In the present invention, the average crystal grain size of ferrite, thedispersion state of cementite, and the texture can be appropriatelycontrolled by controlling the rolling reductions one stand, two stands,and three stands before the final pass of the cold-rolling to 10% ormore and 35% or less each. Thus, the cold-rolling conditions areextremely important. When the rolling reductions one stand, two stands,and three stands before the final pass of the cold-rolling are each lessthan 10%, a shear band is not evenly introduced during cold-rolling, andthus, grain growth varies during recrystallization annealing, resultingin formation of a duplex-grained structure composed of ferrite grainsthat vary in size. As a result, the planar anisotropy of YP isincreased. Meanwhile, when the rolling reductions one stand, two stands,and three stands before the final pass of the cold-rolling each exceed35%, the crystal grain size is reduced during annealing, and thestrength is increased. Moreover, as a result of the increase in thepercentage of the cementite precipitated inside the ferrite grains, theamount of fine cementite inside the ferrite grains increases,deformation of ferrite is suppressed, and the workability is degraded.Thus, the rolling reductions one stand, two stands, and three standsbefore the final pass of cold-rolling are each to be 10% or more and 35%or less.

Difference (|R_(F-1)−R_(F-2)|) between rolling reduction one standbefore final pass Of cold-rolling and rolling reduction two standsbefore the final pass: 10% or less (including 0%)

Difference (|R_(F-2)−R_(F-3)|) between rolling reduction two standbefore final pass of cold-rolling and rolling reduction three standsbefore the final pass: 10% or less (including 0%)

In the present invention, the average crystal grain size of ferrite, thedispersion state of cementite, and the texture can be appropriatelycontrolled by controlling, to 10% or less each, the difference betweenthe rolling reduction one stand before the final pass of thecold-rolling and the rolling reduction two stands before the final passand the difference between the rolling reduction two stands before thefinal pass and the rolling reduction three stands before the final pass.Thus, this condition is one of the extremely important conditions. Whenone or both of the difference between the rolling reduction one standbefore the final pass of the cold rolling and the rolling reduction twostands before the final pass and the difference between the rollingreduction two stands before the final pass and the rolling reductionthree stands before the final pass exceed 10%, a shear band is notevenly introduced during cold-rolling, and thus, grain growth variesduring recrystallization annealing, resulting in formation of aduplex-grained structure composed of ferrite grains that vary in size.As a result, the planar anisotropy of YP is increased. Thus, thedifference between the rolling reduction one stand before the final passof the cold rolling and the rolling reduction two stands before thefinal pass and the difference between the rolling reduction two standsbefore the final pass and the rolling reduction three stands before thefinal pass are each set to be 10% or less. The difference in rollingreduction is preferably 1% or more in any cases, and more preferably 2%or more.

<Method for Producing a Steel Sheet>

A method for producing a steel sheet by using a continuous annealingfurnace and a method for producing a steel sheet by using a boxannealing furnace are described separately below. First, the method thatuses a continuous annealing furnace is described.

A method for producing a steel sheet according to an embodiment of thepresent invention includes heating, in a continuous annealing furnace, acold-rolled full hard steel sheet while setting a dew point to −40° C.or lower in a temperature range of 600° C. or higher and setting anaverage heating rate to 50° C./s or less in a temperature range of 450°C. to T1 temperature−10° C.; holding the heated sheet in a temperaturerange of the T1 temperature or higher and a T2 temperature or lower;cooling the heated sheet to an overaging temperature; and thenperforming an overaging process at a temperature of 300° C. or higherand 550° C. or lower.

Average heating rate in temperature range of 450° C. to T1temperature−10° C.: 50° C./s or less

During heating in continuous annealing, when the average heating rate inthe temperature range of 450° C. to T1 temperature−10° C. exceeds 50°C./s, recrystallization of ferrite is insufficient, and the YP planaranisotropy is increased. Meanwhile, if the average heating rate exceeds50° C./s, the average crystal grain size of ferrite decreases, the yieldstress increases, elongation decreases, and thus the workability isdegraded. Thus, the average heating rate is to be 50° C./s or less. Theaverage heating rate is preferably 40° C./s or less and more preferably30° C./s or less. The lower limit of the average heating rate in thetemperature range of 450° C. to T1 temperature−10° C. is notparticularly limited, but when the average heating rate is less than0.001° C./s, the crystal grain size of ferrite in the steel sheetincreases, and the surface quality may be deteriorated. Thus, theaverage heating rate is preferably 0.001° C./s or more.

T1 temperature (° C.)=735+29×[% Si]−21×[% Mn]+17×[% Cr]

[% X] denotes the mass % of the component element X in the steel sheet,and when that element is not contained, 0 is indicated.

Holding temperature: T1 temperature or higher and T2 temperature orlower

After the heating described above, the sheet is heated to a holdingtemperature equal to or higher than the T1 temperature and equal to orlower than the T2 temperature under desired heating conditions, and heldat that temperature. When the holding temperature (annealingtemperature) in continuous annealing is lower than the T1 temperature,recrystallization does not complete, and the ferrite texture cannot becontrolled. Moreover, since strain due to cold-rolling remains,workability is degraded. In contrast, at an annealing temperatureexceeding the T2 temperature, austenite is generated during annealing, aduplex-grained structure is formed after annealing, the ferrite texturebecomes random, and the YP planar anisotropy is increased. Thus, theholding temperature range in annealing is set to be T1 temperature orhigher and T2 temperature or lower. Holding may be constant-temperatureholding or holding within the above-described temperature range.

T2 temperature (° C.)=960−203×[% C]^(1/2)+45×[% Si]−30×[% Mn]+150×[%Al]−20×[% Cu]+11×[% Cr]+350×[% Ti]+104×[% V]

[% X] denotes the mass % of the component element X in the steel sheet,and when that element is not contained, 0 is indicated.

The holding time for holding is not particularly limited but ispreferably in the range of 10 s or more and 40000 s or less.

After holding, cooling is performed to the overaging temperature. Theaverage cooling rate to the overaging temperature is not particularlylimited, but when the average cooling rate is lower than 10° C./s from680° C. to the overaging temperature, precipitation driving force ofcementite is degraded during overaging. Thus, cementite may notprecipitate sufficiently and the workability may be deteriorated. Thus,the average cooling rate is preferably 10° C./s or more. The averagecooling rate is more preferably 15° C./s or more. This coolingpreferably involves water cooling since the precipitation driving forceof cementite is increased, and most of the cementite can be effectivelyprecipitated in the ferrite grain boundaries. The upper limit of theaverage cooling rate is also not particularly limited, and about 350°C./s is sufficient.

Decarburization from the steel sheet surface during annealing can besuppressed by setting the dew point to −40° C. or lower in thetemperature range of 600° C. or higher, and thus a tensile strength of340 MPa or more prescribed in the present invention can be stablyachieved. When the dew point in the above-described temperature rangeexceeds −40° C., decarburization from the steel sheet surface may causethe strength of the steel sheet to be lower than the standard describedabove. Thus, the dew point in the temperature range of 600° C. or higheris determined to be −40° C. The lower limit of the dew point of theatmosphere is not particularly limited. However, when the dew point islower than −80° C., the effect is saturated, and this poses a costdisadvantage. Thus, the dew point is preferably −80° C. or higher. Thetemperature in the temperature ranges described above is based on thesteel sheet surface temperature. In other words, the dew point isadjusted to be within the above-described range when the steel sheetsurface temperature is within the above-described temperature range.

Overaging temperature range 300° C. or higher and 550° C. or lower

When the temperature during the overaging process is lower than 300° C.,cementite is likely to precipitate inside the ferrite grains. Thus, theoveraging temperature is set to 300° C. or higher. At a temperatureexceeding 550° C., precipitation of cementite becomes difficult. Thus,the temperature during the overaging process is set to be 550° C. orlower. Thus, the overaging temperature range is to be 300° C. or higherand 550° C. or lower. The lower limit of the overaging processtemperature is preferably 360° C. or higher. The upper limit of theoveraging process temperature is preferably 550° C. or lower.

The process time in the overaging temperature range is not particularlylimited, but cementite cannot be sufficiently precipitated if theprocess time is too short. Thus, the process is preferably performed for10 seconds or longer or more preferably 1 minute or longer. The effectsare not compromised even when the process time is long, but from thelimitations related to the production line, 10 minutes is the timeindustrially feasible.

Box annealing may also be employed instead of continuous annealing.Since box annealing involves gradual heating and gradual cooling, nooveraging process is necessary. Moreover, in box annealing, cooling fromthe annealing temperature takes a long time, and there is a sufficienttime for C to diffuse into the ferrite grain boundaries. Thus, theworkability is improved.

Holding temperature in box annealing: 600° C. or higher and 750° C. orlower

When box annealing is performed, un-recrystallized portions remain ifthe holding temperature is lower than 600° C., and coarse grains occurif the holding temperature exceeds 750° C. Thus, the holding temperatureis set to be 600° C. or higher and 750° C. or lower.

Although this feature does not limit the present invention, when theholding time is shorter than 1 hour, soaking inside the coil is notattained, and, when the holding time is 40 hours or longer,decarburization occurs from the steel sheet surface during annealing,and the tensile strength of the steel sheet may decrease to below 340MPa. Thus, the holding time is preferably 1 to 40 hours.

The dew point in the temperature range of 600° C. or higher needs to be−40° C. or lower. The technical significance thereof is the same as thatin the method that uses a continuous annealing furnace, and is omittedfrom the description.

Although the effects of the present invention are not affected, skinpassrolling may be performed after the overaging in continuous annealing orafter box annealing. The skinpass rolling ratio is more preferably 0.5%or more and 1.5% or less since at less than 0.5%, the elongation atyield does not disappear, and at a ratio exceeding 1.5%, the steelbecomes hard.

When the steel sheet is the subject of the trade, the steel sheet isusually cooled to room temperature, and then traded.

<Method for Producing Coated Steel Sheet>

The method for producing a coated steel sheet according to embodimentsof the present invention is the method that involves performing coatingon the steel sheet. Examples of the coating process include agalvanizing process, and a galvannealing process. Annealing andgalvanizing may be continuously performed using one line. Alternatively,the coating layer may be formed by electroplating, such as Zn—Ni alloyelectroplating, or the steel sheet may be coated with hot-dipzinc-aluminum-magnesium alloy. As apparent from the description of thecoating layer described above, Zn is preferable, but a coating processthat uses other metals, such as Al coating, may be performed.

In performing the galvanizing process, the steel sheet is dipped in azinc coating bath at 440° C. or higher and 500° C. or lower to galvanizethe steel sheet, and the coating weight is adjusted by gas wiping or thelike. In galvanizing, a zinc coating bath having an Al content of 0.10mass % or more and 0.23 mass % or less is preferably used. In performingthe galvannealing process, the zinc coating is subjected to an alloyingprocess in a temperature range of 470° C. or higher and 600° C. or lowerafter galvanizing. When the alloying process is performed at atemperature exceeding 600° C., untransformed austenite transforms intopearlite, and the TS may be degraded. Thus, in performing thegalvannealing process, the alloying process is preferably performed in atemperature range of 470° C. or higher and 600° C. or lower. Moreover,an electrogalvanizing process may be performed. The coating weight perside is preferably 20 to 80 g/m² (coating is performed on both sides).The galvannealed steel sheet (GA) is usually subjected to the followingalloying process so as to adjust the Fe concentration in the coatinglayer to 7 to 15 mass %. In the case of typical galvanized steel sheets,the Fe content is less than 7 mass %.

The rolling reduction in skinpass rolling after the coating process ispreferably in the range of 0.1% or more and 2.0% or less. At a rollingreduction less than 0.1%, the effect is small and control is difficult;and thus, 0.1% is the lower limit of the preferable range. At a rollingreduction exceeding 2.0%, the productivity is significantly degraded,and thus 2.0% is the upper limit of the preferable range. Skinpassrolling may be performed on-line or off-line. Skinpass may be performedonce at a targeted rolling reduction, or may be performed in fractionsseveral times.

Other conditions of the production methods are not particularly limited;however, from the productivity viewpoint, a series of processes such asannealing, galvanizing, galvannealing, etc., are preferably performed ina continuous galvanizing line (CGL). After galvanizing, wiping can beperformed to adjust the coating weight. The conditions of the coatingetc., other than the conditions described above may the typicalconditions for galvanization.

EXAMPLES

Steels each having a composition indicated in Table 1 with the balancebeing Fe and unavoidable impurities were melted in a converter, andprepared into slabs by a continuous casting method. The obtained slabwas heated under the conditions indicated in Table 2 and hot-rolled,pickled, and cold-rolled under the conditions indicated in Table 2.

Next, Nos. 1 to 15, 17 to 26, and 30 to 37 indicated in Table 2 weresubjected to a continuous annealing process so as to obtainhigh-strength cold-rolled steel sheets (steel sheets). In Nos. 16 and 27to 29, a box annealing process is performed to obtain high-strengthcold-rolled steel sheets. Note that in Nos. 1, 3, 8, 13, 20, 23, 26, 33,and 36, cooling from the annealing temperature to the overagingtemperature is performed by water cooling.

Some of the high-strength cold-rolled steel sheets were subjected to acoating process so as to obtain galvanized steel sheets (GI),galvannealed steel sheets (GA), electrogalvanized steel sheets (EG),hot-dip zinc-aluminum-magnesium alloy coated steel sheets (ZAM), etc. Azinc bath with Al: 0.14 to 0.19 mass % was used as the galvanizing bathfor GI, and a zinc bath with Al: 0.14 mass % was used for GA. The bathtemperature was 470° C. The coating weight was about 45 to 72 g/m² perside (both sides were coated) for GI and about 45 g/m² per side (bothsides were coated) for GA. In GA, the Fe concentration in the coatinglayer was adjusted to 9 mass % or more and 12 mass % or less. In EG witha Zn—Ni coating layer as the coating layer, the. Ni content in thecoating layer was adjusted to 9 mass % or more and 25 mass % or less. InZAM with a Zn—Al—Mg coating layer as the coating layer, the Al contentin the coating layer was adjusted to 3 mass % or more and 22 mass % orless, and the Mg content was adjusted to 1 mass % or more and 10 mass %or less.

The T1 temperature (° C.) was obtained from the following formula:

T1 temperature (° C.)=735+29×[% Si]−21×[% Mn]+17×[% Cr]

The T2 temperature (° C.) can be calculated as follows:

T2 temperature (° C.)=960−203×[% C]^(1/2)+45×[% Si]−30×[% Mn]+150×[%Al]−20×[% Cu]+11×[% Cr]+350×[% Ti]+104×[% V]

[% X] denotes the mass % of the component element X in the steel sheet,and when that element is not contained, 0 is indicated.

TABLE 1 Steel Composition (mass %) type C Si Mn P S Al N P + S Ti Nb V BCr Mo Cu Ni A 0.120 0.01 0.35 0.014 0.0085 0.044 0.0035 0.022 — — — — —— — — B 0.048 0.03 0.33 0.021 0.0044 0.027 0.0018 0.026 — — — — — — — —C 0.075 0.02 0.47 0.004 0.0084 0.040 0.0034 0.009 — — — — — — — — D0.095 0.02 0.28 0.024 0.0100 0.009 0.0054 0.034 — — — — — — — — E 0.0790.02 0.30 0.031 0.0067 0.038 0.0069 0.037 — — — — — — — — F 0.009 0.020.39 0.004 0.0033 0.045 0.0067 0.007 — — — — — — — — G 0.066 0.26 0.340.035 0.0119 0.037 0.0042 0.047 — — — — — — — — H 0.079 0.01 1.08 0.0150.0138 0.044 0.0038 0.029 — — — — — — — — I 0.034 0.02 0.33 0.110 0.00540.020 0.0063 0.115 — — — — — — — — J 0.048 0.02 0.21 0.026 0.0565 0.0200.0067 0.082 — — — — — — — — K 0.058 0.01 0.24 0.036 0.0120 0.106 0.00170.048 — — — — — — — — L 0.052 0.01 0.37 0.012 0.0086 0.023 0.0034 0.0210.025 — 0.019 — — — — — M 0.065 0.01 0.21 0.015 0.0085 0.039 0.00130.024 — — — 0.0015 — — — — N 0.061 0.01 0.50 0.029 0.0117 0.017 0.00400.041 — — — — 0.41 — 0.18 — O 0.068 0.02 0.36 0.016 0.0118 0.057 0.00190.028 — — — — — 0.09 — — P 0.079 0.02 0.26 0.026 0.0100 0.017 0.00280.036 — — — — — — — 0.07 Q 0.044 0.01 0.26 0.001 0.0022 0.034 0.00510.003 — — — — — — — — R 0.040 0.01 0.21 0.030 0.0068 0.017 0.0031 0.037— 0.039 — — — — — — S 0.067 0.02 0.24 0.005 0.0143 0.029 0.0019 0.020 —0.035 — — — — — — T 0.050 0.01 0.29 0.011 0.0380 0.038 0.0059 0.049 —0.049 — — — — — — U 0.071 0.02 0.35 0.033 0.0136 0.033 0.0020 0.046 — —— — — — — — V 0.048 0.01 0.23 0.018 0.0075 0.027 0.0039 0.025 — — — — —— — — W 0.059 0.01 0.35 0.034 0.0111 0.008 0.0033 0.045 — — — — — — — —T1 T2 temper- temper- Steel Composition (mass %) ature ature type As SbSn Ta Ca Mg Zn Co Zr REM (° C.) (° C.) Remarks A — — — — — — — — — — 728886 Invention steel B — — — — — — — — — — 729 911 Invention steel C — —— — — — — — — — 726 897 Invention steel D — — — — — — — — — — 730 891Invention steel E — — — — — — — — — — 729 901 Invention steel F — — — —— — — — — — 727 937 Comparative steel G — — — — — — — — — — 735 915Comparative steel H — — — — — — — — — — 713 878 Comparative steel I — —— — — — — — — — 728 916 Comparative steel J — — — — — — — — — — 731 913Comparative steel K — — — — — — — — — — 730 921 Comparative steel L — —— — — — — — — — 727 917 Invention steel M — — — — — — — — — — 731 908Invention steel N — — — — — — — — — — 732 899 Invention steel O — 0.005— — — — — — — 728 906 Invention steel P 0.005 — 0.005 — — — — — — — 730898 Invention steel Q — — — 0.005 — — — — — — 730 915 Invention steel R— 0.006 — — — — — — — — 731 916 Invention steel S — — 0.004 — — — — — —— 730 905 Invention steel T — — — 0.006 — — — — — — 729 912 Inventionsteel U — — — — 0.0028 — — — 0.004 — 728 901 Invention steel V — — — — —0.0041 0.009 0.005 — — 730 913 Invention stell W — — — — — — — — —0.0031 728 902 Invention steel

TABLE 2 Average cooling rate from Pass after Finish- immedi- Finish-finish Annealing conditions rolling ately rolling rolling RollingAnnealing Average Over- inlet before delivery to coiling Coilingreduction method Drew heating Annealing Whether aging Type of temper-final Final temper- temper- temper- in cold- |RF₋₁- |RF₋₂- (*) pointrate temper- water temper- Presence coating Steel ature pass pass atureature ature rolling R_(F-1) R_(F-2) R_(F-3) RF₋₂| RF₋₃| (CAL or *1 *2ature cooling is ature of coating etc. No. type (° C.) (%) (%) (° C.) (°C.) (° C.) (%) (%) (%) (%) (%) (%) BAF) (° C.) (° C./s) (° C.) performed(° C.) (Yes/No) (**) Remarks  1 A 1060 20 10 870 35 680 70 31 25 22 6 3CAL −45 10 810 Yes 440 No CR Example  2 B 1050 22 11 880 28 660 73 31 2621 5 5 CAL −50 15 770 No 420 Yes GA Example  3 C 1040 20 10 890 13 63070 30 24 21 6 3 CAL −47 20 830 Yes 460 Yes GI Example  4 C  980 21 11910 23 630 71 29 23 20 6 3 CAL −46 25 790 No 450 Yes GA ComparativeExample  5 C 1110 18  3 890 19 600 64 29 24 19 5 5 CAL −42 20 850 No 360No CR Comparative Example  6 C 1020 20 12 770 15 590 70 31 28 22 3 6 CAL−47 10 820 No 400 Yes EG Comparative Example  7 C 1030 23 13 870  4 55070 30 26 20 4 6 CAL −47 15 750 No 440 No CR Comparative Example  8 C1150 22 10 850 12 820 70 30 25 19 5 6 CAL −48 10 750 Yes 460 Yes GAComparative Example  9 C 1060 21 11 880 10 630 35 11 10 10 1 0 CAL −4815 770 No 470 Yes EG Comparative Example 10 C 1050 22 10 910 18 610 7537 28 20 9 8 CAL −43 20 800 No 460 Yes GA Comparative Example 11 C 102023 12 860 20 620 73 34 22 20 12 2 CAL −45 25 800 No 470 No CRComparative Example 12 C 1160 22 11 920 22 700 65 29 24 19 5 5 CAL −3522 825 No 480 Yes GI Comparative Example 13 C 1030 19 10 910 25 610 7530 25 20 5 5 CAL −47 54 800 Yes 440 No CR Comparative Example 14 C 115022 13 900 15 600 75 31 24 22 7 2 CAL −47 15 675 No 460 No CR ComparativeExample 15 C 1060 19 12 880 21 650 75 30 25 21 5 4 CAL −47 10 920 No 400Yes GA Comparative Example 16 C 1050 23 12 880 16 650 75 31 28 19 3 9BAF −47 — 575 — — Yes GI Comparative Example 17 C 1040 20 11 890 13 70080 32 26 20 6 6 CAL −47 15 800 No 250 Yes GA Comparative Example 18 D1060 21 11 900 23 680 95 34 24 22 10 2 CAL −50 20 750 No 400 Yes GAExample 19 E 1040 20 10 890 26 690 90 33 28 20 5 8 CAL −48 15 800 No 420Yes GA Example 20 F 1160 23 10 940 25 570 80 32 25 21 7 4 CAL −47 15 780Yes 400 Yes GA Comparative Example 21 G 1050 23 10 860 45 590 80 32 2819 4 9 CAL −47 15 750 No 420 No CR Comparative Example 22 H 1060 22 13890 21 720 75 31 24 22 7 2 CAL −49 20 780 No 440 Yes GI ComparativeExample 23 I 1060 19 11 880 19 640 65 27 25 20 2 5 CAL −48 20 760 Yes400 Yes GI Comparative Example 24 J 1150 23 12 860 32 690 65 28 25 20 35 CAL −47 15 750 No 400 No CR Comparative Example 25 K 1160 19 13 860 34600 65 27 26 21 1 5 CAL −40 10 760 No 500 Yes GA Comparative Example 26L 1040 23 13 920 18 610 65 28 25 22 3 3 CAL −41 34 750 Yes 470 Yes GIExample 27 M 1060 21 11 910 20 700 72 31 24 19 7 5 BAF −40 — 750 — — YesGI Example 28 N 1050 19 12 880 21 660 68 30 25 22 5 3 BAF −42 — 620 — —No CR Example 29 O 1060 23 12 859 10 600 75 30 28 20 2 8 BAF −42 — 675 —— Yes GI Example 30 P 1030 22 13 910 14 570 75 29 24 21 5 3 CAL −52 15815 No 400 Yes GA Example 31 Q 1160 22 11 890 18 640 75 30 26 22 4 4 CAL−50  2 750 No 420 No CR Example 32 R 1050 21 10 850 19 600 75 29 25 19 46 CAL −51 10 770 No 380 Yes ZAM Example 33 S 1060 20 11 840 11 620 58 2221 21 1 0 CAL −50 15 750 Yes 400 No CR Example 34 T 1040 22 10 900 17530 65 28 28 22 0 6 CAL −49 20 830 No 440 Yes GA Example 35 U 1150 20 10890  8 630 70 29 24 21 5 3 CAL −40 15 800 No 440 Yes EG Example 36 V1030 23 13 820 14 670 70 30 25 21 5 4 CAL −48 10 750 Yes 420 Yes GIExample 37 W 1060 22  9 870 18 630 70 29 24 22 5 2 CAL −47 20 760 No 460Yes GA Example (*) CAL: continuous annealing, BAF: box annealing (**)CR: cold-rolled steel sheet (not coated), GI: galvanized steel sheet(not subjected to galvannealing), GA: galvannealed steel sheet, EG:electrogalvanized steel sheet, ZAM: hot-dip zinc-aluminum-magnesiumalloy coated steel sheet *1 Dew point in furnace in a temperature rangeof 600° C. or higher *2 Average heating rate in a temperature range of450° C. to [T1 temperature-10° C.]

The high-strength cold-rolled steel sheets and the high-strength coatedsteel sheets obtained as above were used as sample steels to evaluatetheir mechanical properties. The mechanical properties were evaluated bythe following tensile test. The results are indicated in Table 3. Thesheet thickness of the each steel sheet, which is a sample steel sheet,is also indicated in Table 3.

JIS No. 5 test pieces taken so that the longitudinal direction of thetest pieces was in three directions, namely, the rolling direction (Ldirection) of the steel sheet, a direction (D direction) 45° withrespect to the rolling direction of the steel sheet, and a direction (Cdirection) 900 with respect to the rolling direction of the steel sheet,were used to perform a tensile test in accordance with JIS Z 2241(2011), and the YP (yield stress), the TS (tensile strength), and El(total elongation) were measured. YP, TS, and El indicated in Table 3are the measurement results of the test pieces taken in the C direction.|ΔYP| was calculated by the above-described calculation method.

For the purposes of the present invention, the workability was evaluatedas satisfactory when the product, TS×El (El denotes the totalelongation), was 13000 MPa % or more. The YP planar anisotropy wasevaluated as satisfactory when the value of |ΔYP|, which is an index ofthe YP planar anisotropy, was 30 MPa or less.

The steel sheets were evaluated as having excellent surface propertieswhen the scale defect length incidence per 100 coils was 1.2% or less.The scale defect length incidence is determined by formula (2) below,and the surface properties were observed with a surface tester andevaluated as “excellent” when the scale defect length incidence per 100coils was 0.3% or less, “fair” when the incidence was more than 0.3% butnot more than 1.2%, and “poor” when the incidence was more than 1.2%.

(Scale defect length incidence)=(total length of defects determined tobe scale defects in L direction)/(delivery-side coil length)×100  (2)

According to the methods described above, the inverse intensity ratio ofthe γ-fiber to the α-fiber in the ferrite texture at a position ¼ of thethickness of the steel sheet, the ferrite average crystal grain size,and the proportion of the cementite present in the grain boundaries weredetermined.

The coatability was evaluated as satisfactory when the bare spot defectincidence per 100 coils was 0.8% or less. The coating defect lengthincidence is determined by formula (3) below, and the surface propertieswere observed with a surface tester and evaluated as “excellent” whenthe scale defect length incidence per 100 coils was 0.2% or less, “fair”when the incidence was more than 0.2% but not more than 0.8%, and “poor”when the incidence was more than 0.8%.

(Coating defect length incidence)=(total length of defects determined tobe bare defects in L direction)/(delivery-side coil length)×100  (3)

The results are indicated in Table 3. As indicated in Table 3, inExamples of the present invention, TS was 340 MPa or more, theworkability was excellent, the YP planar anisotropy was excellent, andthe surface properties were excellent. In contrast, in ComparativeExamples, at least one of the strength, the balance between strength andductility, the YP planar anisotropy, and the scale-induced defectscontamination ratio per 50 coils was not satisfactory.

Although the embodiments of the present invention are describedheretofore, the present invention is not limited by the description ofthe embodiments, which constitutes part of the disclosure of the presentinvention. In other words, other embodiments, examples, andimplementation techniques practiced by a person skilled in the art andthe like on the basis of the embodiments are all within the scope of thepresent invention. For example, in a series of heat treatments in theproduction methods described above, the facilities in which the steelsheet is heat-treated and the like are not particularly limited as longas the heat history conditions are satisfied.

TABLE 3 Scale F Grain defect average boundary γ-Fiber- length crystalpercentage to-α-fiber incidence grain of precip- inverse per 100 Steelsize itated θ intensity YP TS EI TS × EI |ΔYP| coils Surface No. type(μm) (%) ratio in F (MPa) (MPa) (%) (MPa · %) (MPa) (%) propertiesCoatability Remarks  1 A 8.4 88 4.1 241 383 39.3 15052 19 0.1 Excellent— Example  2 B 15.9 79 3.5 264 370 38.9 14393 6 0.0 Excellent FairExample  3 C 17.9 90 4.2 208 366 41.6 15226 15 0.7 Fair Fair Example  4C 18.5 90 0.7 265 372 37.5 13950 33 0.8 Fair Fair Comparative Example  5C 27.5 80 0.6 190 335 44.6 14941 36 0.3 Fair Fair Comparative Example  6C 18.4 66 0.7 214 371 30.2 11204 40 0.8 Fair Fair Comparative Example  7C 18.0 36 5.2 251 357 32.6 11638 16 1.0 Fair — Comparative Example  8 C16.0 37 2.7 253 367 32.6 11964 17 0.8 Fair Fair Comparative Example  9 C16.2 35 0.6 206 373 43.8 16337 36 0.8 Fair Fair Comparative Example 10 C4.2 38 3.2 253 393 28.7 11279 20 0.2 Excellent Fair Comparative Example11 C 20.5 85 0.7 280 381 39.5 15050 35 0.5 Fair — Comparative Example 12C 24.2 89 4.6 217 305 49.5 15098 19 1.0 Fair Fair Comparative Example 13C 3.7 75 0.6 293 376 32.3 12145 36 0.8 Fair — Comparative Example 14 C8.1 80 0.7 296 381 33.0 12573 36 0.6 Fair — Comparative Example 15 C19.4 67 0.6 202 366 41.8 15299 35 1.1 Poor Fair Comparative Example 16 C9.2 69 0.7 283 387 29.8 11533 39 0.5 Fair Excellent Comparative Example17 C 19.3 34 3.8 207 369 30.6 11291 9 0.5 Fair Excellent ComparativeExample 18 D 9.6 72 4.2 271 380 41.8 15884 13 0.2 Excellent ExcellentExample 19 E 21.1 66 4.8 207 371 43.3 16064 10 0.3 Excellent FairExample 20 F 24.7 85 5.3 184 321 48.1 15440 12 0.5 Fair ExcellentComparative Example 21 G 21.5 36 2.5 213 351 36.4 12776 20 1.8 Poor —Comparative Example 22 H 15.5 35 3.1 215 352 32.9 11581 20 0.1 ExcellentFair Comparative Example 23 I 24.4 34 3.7 242 345 34.5 11903 7 0.1Excellent Fair Comparative Example 24 J 3.1 62 4.1 266 379 31.0 11749 170.2 Excellent — Comparative Example 25 K 4.4 62 3.3 263 383 32.6 1248618 0.1 Excellent Fair Comparative Example 26 L 10.6 75 2.7 246 381 40.515431 7 0.0 Excellent Excellent Example 27 M 21.8 76 2.9 207 358 42.115072 13 0.2 Excellent Excellent Example 28 N 8.0 82 4.3 252 384 41.716013 20 0.2 Excellent — Example 29 O 13.8 65 2.6 246 385 41.5 15978 70.2 Excellent Excellent Example 30 P 12.2 70 5.5 272 386 42.2 16289 140.0 Excellent Fair Example 31 Q 10.8 85 2.9 253 387 38.5 14900 13 0.8Fair — Example 32 R 8.3 73 2.7 252 386 37.2 14359 15 0.1 Excellent FairExample 33 S 8.5 89 4.2 244 389 37.4 14549 17 0.1 Excellent — Example 34T 12.7 72 3.0 253 384 42.0 16128 17 0.1 Excellent Excellent Example 35 U12.2 71 4.3 264 388 37.2 14434 18 0.0 Excellent Fair Example 36 V 9.1 892.6 255 383 41.9 16048 6 0.1 Excellent Excellent Example 37 W 11.9 844.3 236 385 36.8 14168 19 0.1 Excellent Fair Example F ferrite, θcementite

INDUSTRIAL APPLICABILITY

According to embodiments of the present invention, a steel sheet thathas a TS of 340 MPa or more, excellent workability, excellent YP planaranisotropy, and excellent surface properties, and the like can beproduced. Moreover, when the steel sheet and the like obtained accordingto the production method of the present invention are applied to, forexample, automobile structural elements, fuel efficiency can be improvedthrough car body weight reduction, and thus the present invention offersconsiderable industrial advantages.

1-9. (canceled)
 10. A steel sheet comprising: a composition containing,in terms of mass %, C: 0.010% or more and 0.150% or less, Si: 0.20% orless, Mn: 1.00% or less, P: 0.100% or less, S: 0.0500% or less, Al:0.001% or more and 0.100% or less, N: 0.0100% or less, and the balancebeing Fe and unavoidable impurities, in which 0.002%≤[% P]+[% S]≤0.070%([% M] denotes a content (mass %) of M element in steel) is satisfied; asteel structure in which ferrite has an average crystal grain size of 5μm or more and 25 μm or less, 40% or more of cementite in terms of areafraction is precipitated in ferrite grain boundaries, and the ferritehas a texture in which an inverse intensity ratio of γ-fiber to α-fiberis 0.8 or more and 7.0 or less; and a tensile strength of 340 MPa ormore.
 11. The steel sheet according to claim 10, wherein the compositionfurther contains, in terms of mass %, at least one element selectedfrom: Ti: 0.001% or more and 0.100% or less, Nb: 0.001% or more and0.100% or less, V: 0.001% or more and 0.100% or less, B: 0.0001% or moreand 0.0050% or less, Cr: 0.01% or more and 1.00% or less, Mo: 0.01% ormore and 0.50% or less, Cu: 0.01% or more and 1.00% or less, Ni: 0.01%or more and 1.00% or less, As: 0.001% or more and 0.500% or less, Sb:0.001% or more and 0.200% or less, Sn: 0.001% or more and 0.200% orless, Ta: 0.001% or more and 0.100% or less, Ca: 0.0001% or more and0.0200% or less, Mg: 0.0001% or more and 0.0200% or less, Zn: 0.001% ormore and 0.020% or less, Co: 0.001% or more and 0.020% or less, Zr:0.001% or more and 0.020% or less, and REM: 0.0001% or more and 0.0200%or less.
 12. A coated steel sheet comprising the steel sheet accordingto claim 10, and a coating layer on a surface of the steel sheet.
 13. Acoated steel sheet comprising the steel sheet according to claim 11, anda coating layer on a surface of the steel sheet.
 14. A method forproducing a hot-rolled steel sheet, the method comprising heating asteel slab having the composition described in claim 10; rough-rollingthe heated steel slab; in subsequent finish rolling, hot-rolling therough-rolled steel slab under conditions of a finish-rolling inlettemperature of 1020° C. or higher and 1180° C. or lower, a rollingreduction in a final pass of the finish rolling of 5% or more and 15% orless, a rolling reduction in a pass before the final pass of 15% or moreand 25% or less, and a finish-rolling delivery temperature of 800° C. orhigher and 1000° C. or lower; after the hot rolling, cooling thehot-rolled sheet to a coiling temperature at an average cooling rate of5° C./s or more and 90° C./s or less; and coiling the cooled sheet at acoiling temperature of 400° C. or higher and 800° C. or lower.
 15. Themethod for producing a hot-rolled steel sheet according to claim 14,wherein the composition further contains, in terms of mass %, at leastone element selected from: Ti: 0.001% or more and 0.100% or less, Nb:0.001% or more and 0.100% or less, V: 0.001% or more and 0.100% or less,B: 0.0001% or more and 0.0050% or less, Cr: 0.01% or more and 1.00% orless, Mo: 0.01% or more and 0.50% or less, Cu: 0.01% or more and 1.00%or less, Ni: 0.01% or more and 1.00% or less, As: 0.001% or more and0.500% or less, Sb: 0.001% or more and 0.200% or less, Sn: 0.001% ormore and 0.200% or less, Ta: 0.001% or more and 0.100% or less, Ca:0.0001% or more and 0.0200% or less, Mg: 0.0001% or more and 0.0200% orless, Zn: 0.001% or more and 0.020% or less, Co: 0.001% or more and0.020% or less, Zr: 0.001% or more and 0.020% or less, and REM: 0.0001%or more and 0.0200% or less.
 16. A method for producing a cold-rolledfull hard steel sheet, the method comprising pickling a hot-rolled steelsheet obtained in the method according to claim 14; and performingcold-rolling at a rolling reduction of 55% or more, wherein, when arolling reduction in a final pass of cold rolling is assumed to be R_(F)and rolling reductions one stand, two stands, and three stands beforethe final pass are respectively assumed to be R_(F-1), R_(F-2), andR_(F-3), the rolling reductions R_(F-1), R_(F-2), and R_(F-3) are eachset to 10% or more and 35% or less, a difference (|R_(F-1)−R_(F-2)|)between the rolling reduction one stand before the final pass and therolling reduction two stands before the final pass is set to 10% orless, and a difference (|R_(F-2)−R_(F-3)|) between the rolling reductiontwo stands before the final pass and the rolling reduction three standsbefore the final pass is set to 10% or less.
 17. A method for producinga cold-rolled full hard steel sheet, the method comprising pickling ahot-rolled steel sheet obtained in the method according to claim 15; andperforming cold-rolling at a rolling reduction of 55% or more, wherein,when a rolling reduction in a final pass of cold rolling is assumed tobe R_(F) and rolling reductions one stand, two stands, and three standsbefore the final pass are respectively assumed to be R_(F-1), R_(F-2),and R_(F-3), the rolling reductions R_(F-1), R_(F-2), and R_(F-3) areeach set to 10% or more and 35% or less, a difference(|R_(F-1)−R_(F-2)|) between the rolling reduction one stand before thefinal pass and the rolling reduction two stands before the final pass isset to 10% or less, and a difference (|R_(F-2)−R_(F-3)|) between therolling reduction two stands before the final pass and the rollingreduction three stands before the final pass is set to 10% or less. 18.A method for producing a steel sheet, the method comprising heating, ina continuous annealing furnace, a cold-rolled full hard steel sheet,which is obtained in the method according to claim 16, while setting adew point to −40° C. or lower in a temperature range of 600° C. orhigher and setting an average heating rate to 50° C./s or less in atemperature range of 450° C. to T1 temperature−10° C.; holding the sheetin a temperature range of the T1 temperature or higher and a T2temperature or lower; cooling the heated sheet to an overagingtemperature; and then performing an overaging process at a temperatureof 300° C. or higher and 550° C. or lower, where:T1 temperature (° C.)=735+29×[% Si]−21×[% Mn]+17×[% Cr]T2 temperature (° C.)=960−203×[% C]^(1/2)+45×[% Si]−30×[% Mn]+150×[%Al]−20×[% Cu]+11×[% Cr]+350×[% Ti]+104×[% V] where in the formulaeabove, [% X] denotes a content (mass %) of a component element X in thesteel sheet, and 0 is indicated when the element is not contained.
 19. Amethod for producing a steel sheet, the method comprising heating, in acontinuous annealing furnace, a cold-rolled full hard steel sheet, whichis obtained in the method according to claim 17, while setting a dewpoint to −40° C. or lower in a temperature range of 600° C. or higherand setting an average heating rate to 50° C./s or less in a temperaturerange of 450° C. to T1 temperature−10° C.; holding the sheet in atemperature range of the T1 temperature or higher and a T2 temperatureor lower; cooling the heated sheet to an overaging temperature; and thenperforming an overaging process at a temperature of 300° C. or higherand 550° C. or lower, where:T1 temperature (° C.)=735+29×[% Si]−21×[% Mn]+17×[% Cr]T2 temperature (° C.)=960−203×[% C]^(1/2)+45×[% Si]−30×[% Mn]+150×[%Al]−20×[% Cu]+11×[% Cr]+350×[% Ti]+104×[% V] where in the formulaeabove, [% X] denotes a content (mass %) of a component element X in thesteel sheet, and 0 is indicated when the element is not contained. 20.The method for producing a steel sheet according to claim 18, whereinthe cooling to the overaging temperature is performed by water cooling.21. The method for producing a steel sheet according to claim 19,wherein the cooling to the overaging temperature is performed by watercooling.
 22. A method for producing a steel sheet, the method comprisingheating and holding a cold-rolled full hard steel sheet, which isobtained in the method according to claim 16, to and in a temperaturerange of 600° C. or higher and 750° C. or lower in a box annealingfurnace while setting a dew point to −40° C. or lower in a temperaturerange of 600° C. or higher.
 23. A method for producing a steel sheet,the method comprising heating and holding a cold-rolled full hard steelsheet, which is obtained in the method according to claim 17, to and ina temperature range of 600° C. or higher and 750° C. or lower in a boxannealing furnace while setting a dew point to −40° C. or lower in atemperature range of 600° C. or higher.
 24. A method for producing acoated steel sheet, the method comprising coating the steel sheetobtained in the method according to claim
 18. 25. A method for producinga coated steel sheet, the method comprising coating the steel sheetobtained in the method according to claim
 19. 26. A method for producinga coated steel sheet, the method comprising coating the steel sheetobtained in the method according to claim
 20. 27. A method for producinga coated steel sheet, the method comprising coating the steel sheetobtained in the method according to claim
 21. 28. A method for producinga coated steel sheet, the method comprising coating the steel sheetobtained in the method according to claim
 22. 29. A method for producinga coated steel sheet, the method comprising coating the steel sheetobtained in the method according to claim 23.